Remember a 7th chord is a triad that has been extended to include the 7th note of the scale. This creates a 4-note chord. It is used to make richer harmonic progressions. There are 5 essential 7th chords. We have covered three of them. The other two are:

Half Diminished 7th Chord or Minor7b5

Diminished 7th Chord

The Minor7b5 is formed by a root note, flat 3rd, diminished 5th and flat or minor 7th.